Railroad Carrier Employee 

Exposure to Radiation 

Report to Congress 

September 2010 

U.S. Department of Transportation 

Federal Railroad Administration 

Office of Railroad Safety
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i

Contents

The Mandate.................................................................................................................................. ii

Executive Summary ...................................................................................................................... 1

Background ................................................................................................................................... 3

Definitions and Discussion of Technical Terms and Concepts .................................................. 3

Measuring Radiation ................................................................................................................... 5

Human Exposure to Radiation .................................................................................................... 7

Risks Associated with Radiation Exposure ................................................................................ 7

Latency and Other Sources of Damage ...................................................................................... 8

Radiation Safety Practice ........................................................................................................... 11

Protection from Exposure to Radiation ..................................................................................... 12

Regulatory Regimes and Exposure Limits ............................................................................... 15

Nuclear Regulatory Commission Standards and Limits ........................................................... 15

Occupational Safety and Health Administration Standards and Limits ................................... 16

Previous Research and Predicted Levels of Exposure............................................................. 19

Source Strength ......................................................................................................................... 19

Train Crew and Escorts ............................................................................................................. 23

Car Inspectors and Close Proximity Yard Workers .................................................................. 23

Rail Yard Workers .................................................................................................................... 24

Conclusions about the DTS Exposure Estimates ...................................................................... 24

Real-World Exposures................................................................................................................ 25

U.S. Railroad Data–Norfolk Southern Railway ........................................................................ 25

The French Study ...................................................................................................................... 27

The German Study .................................................................................................................... 30

Conclusions in Response to the Congressional Mandates ....................................................... 33

Appendix A ................................................................................................................................ A-1

Appendix B ................................................................................................................................ B-1
ii

The Mandate 

Section 411 of the Federal Railroad Safety Improvement Act of 2008 (Pub. L. No. 110-432) 

reads as follows: 

(a) STUDY. The Secretary of Transportation shall, in consultation with the 

Secretary of Energy, the Secretary of Labor, the Administrator of the 

Environmental Protection Agency, and the Chairman of the Nuclear Regulatory 

Commission, as appropriate, conduct a study of the potential hazards to which 

employees of railroad carriers and railroad contractors or subcontractors are 

exposed during the transportation of high-level radioactive waste and spent 

nuclear fuel (as defined in section 5101(a) of title 49, United States Code), 

supplementing the report submitted under section 5101(b) of that title, which may 

include— 

(1) an analysis of the potential application of “as low as reasonably achievable” 

principles for exposure to radiation to such employees with an emphasis on the 

need for special protection from radiation exposure for such employees during the 

first trimester of pregnancy or who are undergoing or have recently undergone 

radiation therapy; 

(2) the feasibility of requiring real-time dosimetry monitoring for such employees; 

(3) the feasibility of requiring routine radiation exposure monitoring in fixed 

railroad locations, such as yards and repair facilities; and 

(4) a review of the effectiveness of the Department’s packaging requirements for 

radioactive materials. 

(b) REPORT. Not later than 18 months after the date of enactment of this Act, 

the Secretary of Transportation shall transmit a report on the results of the study 

required by subsection (a) and any recommendations to further protect employees 

of a railroad carrier or of a contractor or subcontractor to a railroad carrier from 

unsafe exposure to radiation during the transportation of high-level radioactive 

waste and spent nuclear fuel to the Senate Committee on Commerce, Science, and 

Transportation and the House of Representatives Committee on Transportation 

and Infrastructure. 

(c) REGULATORY AUTHORITY. The Secretary of Transportation may issue 

regulations that the Secretary determines appropriate, pursuant to the report 

required by subsection (b), to protect railroad employees from unsafe exposure to 

radiation during the transportation of radioactive materials. 

 
1

Executive Summary 

This report was prepared by the Federal Railroad Administration (FRA) following the specific 

mandate of Congress to investigate occupational exposures to ionizing radiation of specific 

groups of employees during railroad transportation of high-level radioactive waste (HLRW) and 

spent nuclear fuel (SNF). Commercial shipments of these types of materials are very rare since 

transportation to the Yucca Mountain Repository1 is not being conducted at this time. 

In an effort to establish the known levels of exposure to the materials in question, FRA obtained 

exposure information from one of the Class I railroads that has been conducting exposure 

monitoring during shipments of SNF materials. In addition, FRA reviewed reports of exposure 

assessments conducted in foreign locations where transportation by rail is occurring more 

frequently. 

Both of these sources indicate that the different classes of workers identified in the mandate were 

found to have levels of exposure significantly lower than those expected, and also significantly 

lower than the radiation exposure dose limits established by the Occupational Safety and Health 

Administration (OSHA) in Title 29 Code of Federal Regulations (CFR) Section 1910.1096. 

The data presented in this report include theoretical predictions of potential exposure to 

radiation, as well as real-world exposure assessments in the United States and two European 

countries. Both the theoretical findings and the real-world experience indicate that potential and 

actual exposures are well below the currently established permissible levels. All of the current 

regulatory permissible levels have been established recognizing the importance of the “as low as 

reasonably achievable” principles in minimizing exposures. 

The real-world exposures in the three studies cited all found exposures well below regulatory 

limits. These facts would indicate that female employees exposed during the first trimester of 

pregnancy would not likely face a risk of adverse health effects to themselves or the fetus. 

The medical and health implications of someone undergoing radiation therapy and the interaction 

with on-the-job exposures cannot be presumed, given the complexity of the medical procedures 

and types of therapy available in today’s practice of medicine. 

In response to the question of the feasibility of requiring routine radiation exposure monitoring in 

fixed railroad locations, such as yards and repair facilities, the study found that the use of 

dedicated trains results in minimal dwell time in such locations, and the already significant 

monitoring of the packages of HLRW and SNF required by regulation, along with the known 

shielding properties of the packages, would make monitoring of these types of sites redundant 

and unnecessarily costly, and would serve no practical purpose. 

 

1

 The Yucca Mountain Repository is the United States’ designated geological repository storage facility for spent 

nuclear reactor fuel and other radioactive waste. It is located between the Mohave and the Great Basin Deserts in 

Nevada. 
2

The real-world levels measured by Norfolk Southern Railway (NS) during actual shipments 

indicate that the packaging far exceeds the minimum requirements for shielding, thereby 

providing an extra margin of safety for employees and the general public. 

The U.S. Department of Transportation (DOT) does not believe that any regulatory action is 

necessary at this time to further protect railroad employees from unsafe exposure to radiation 

during the transportation of radioactive materials. 

In preparing this report, FRA coordinated closely with the DOT Pipeline and Hazardous 

Materials Safety Administration (PHMSA), which also issues regulations governing the 

transportation of hazardous materials in all modes,2

 and with DOT’s Office of the Secretary. In 

addition, FRA consulted with the U.S. Department of Energy (DOE), the Environmental 

Protection Agency, the Nuclear Regulatory Commission (NRC), and OSHA. 

The transportation of SNF/HLRW is thoroughly regulated, and several Government agencies 

play active, highly coordinated roles to ensure its safety. Over the past 45 years, approximately 

600 train movements of these materials have occurred by rail without any incidents affecting the 

integrity of the shipping packages. At the discretion of the shipper or carrier parties involved, a 

majority of these shipments were made using “special” or dedicated trains.3

 The responsible 

agencies work continually to verify the safety of packaging, rolling stock, and procedures, and 

oversee the training of personnel involved in transportation. 

The railroad industry also issued its own standard for movement of these commodities, that seeks 

to establish performance guidelines for a cask/car/train system transporting high-level 

radioactive material. These guidelines are designed to ensure safe transportation, minimize time 

in transit, and incorporate the best available technology to minimize the potential for rail 

accidents.

During previous work involving these materials, specifically the report to Congress titled “Use of 

Dedicated Trains for Transportation of High-Level Radioactive Waste and Spent Nuclear Fuel” 

(March 2005), also called the Dedicated Train Study (DTS) conducted under a prior mandate,4

the safety and integrity of the packaging and shipments was reported. This report will reference 

the DTS to the extent that it contains information contributing to the understanding of 

occupational exposures, which are the subject of this mandate. 

 

2

 FRA and PHMSA develop hazardous materials regulations specifically applicable to the rail mode for issuance by 

PHMSA. FRA enforces hazardous materials regulations applicable to transportation by rail. Both agencies act by 

delegation from the Secretary of Transportation. 

3

 As used in this report, a “special” or “dedicated” train is a train that consists only of equipment and lading 

associated with the transportation of SNF/HLRW. That is, the train consists only of necessary motive power, buffer 

cars, and cask car or cars, together with a car for escort personnel. Such a train does not transport other rail rolling 

stock, other revenue freight, or other company freight. 

4

 Section 15 of the Hazardous Materials Transportation Uniform Safety Act of 1990 (Pub. L. No. 101-615), 

amended Section 116 of the Hazardous Materials Transportation Act (49 U.S.C. App. 1813). 
3

Background 

Definitions and Discussion of Technical Terms and Concepts 

A number of technical terms are used to describe the measurement of and exposure to radiation. 

Since these terms are used throughout the report, it will be helpful to begin with definitions and a 

brief discussion of these terms and concepts. 

Radiation is energy that is emitted or transmitted in the form of rays, waves, or particles. Radio 

waves, light, and heat are forms of radiation. These are low-energy forms of radiation, and are 

considered non-ionizing radiation. 

Ionizing radiation is radiation that has enough energy to remove electrons from atoms or 

molecules (groups of atoms) when it passes through or collides with another material. In the 

process called ionization, an atom or molecule loses an electron, which results in the formation 

of a charged atom (or molecule) that is called an ion. The amount of ionization depends on the 

level of energy of the impinging individual particles or waves, not their number. A large number 

of particles or waves with low energy will not cause ionization. 

Note: It is assumed that the term “radiation,” as used in the congressional mandate, means 

ionizing radiation. Therefore, in this report, the use of the word “radiation” means ionizing 

radiation. Many types and sources of both ionizing and non-ionizing radiation are present in the 

railroad environment when shipments of HLRW and SNF are moved. 

Ionizing radiation can take the form of subatomic particles or electromagnetic waves. The two 

primary types of ionizing particles are alpha particles and beta particles. 

Alpha particles consist of two protons and two neutrons. They are relatively heavy, 

high-energy particles, with a positive charge of +2 from its two protons. Because of their 

large mass and electric charge, alpha particles travel relatively slowly in air and rapidly 

lose energy. They are easily stopped by a piece of paper or by coming in contact with 

human skin. 

Beta particles are free electrons. They have a very low mass, about 1/2000 of the mass 

of a proton or neutron. Due to the small mass, the amount of ionization that beta particles 

can cause depends on the energy level imparted to them when they are created by the 

decay of radioactive materials such as tritium, carbon-14, and other similar substances. 

Beta particles can travel several feet in open air. They are easily stopped by solid 

materials such as sheets of aluminum, glass, or plexiglass. 

There are two types of photon-ionizing (pure energy) radiation: gamma rays and x-rays. 

Gamma radiation is very high-energy ionizing radiation that has about 10,000 times as 

much energy as the photons in the visible range of the electromagnetic spectrum (visible 

light). Gamma photons are pure electromagnetic energy, thus, they have no mass and no 

electrical charge. 
4

X-radiation is also very high-energy ionizing radiation, similar to gamma radiation, but 

generally has lower wave lengths and energy levels—although the ranges of energy and 

wavelength overlap for both types of radiation. The primary difference between the two 

types is where in the atom the energy waves originate; for gamma rays it is the nucleus, 

for x-rays it is the electrons. X-ray photons are also pure electromagnetic energy, 

therefore, they have no mass and no electrical charge. Since these types possess such 

high energy levels, the materials used to shield against them must be very dense, such as 

steel or lead. 

Since the focus of this report is “the potential hazards to which employees are exposed during the 

transportation of high-level radioactive waste and spent nuclear fuel,” the terms are defined 

below. 

SNF is fuel that has been withdrawn from a nuclear reactor following irradiation and has 

undergone at least 1 year’s decay since being used as a source of energy in a power 

reactor. Further, reprocessing has not separated the constituent elements of the SNF. 

This fuel includes: 

1) Intact, non-defective fuel assemblies 

2) Failed fuel assemblies in canisters 

3) Fuel assemblies in canisters 

4) Consolidated fuel rods in canisters 

5) Non-fuel components inserted in pressurized water reactor fuel assemblies 

6) Fuel channels attached to boiling water reactor fuel assemblies 

7) Non-fuel components and structural parts of assemblies in canisters [42 U.S.C. 

10101(23), 40 CFR 191.02, and DOE Order 5820.2A] 

HLRW results from the reprocessing of SNF in a commercial or defense facility. It 

includes liquid waste produced directly in reprocessing, and any solid waste derived from 

the liquid that contains a combination of transuranic waste and fission products in 

concentrations requiring permanent isolation [42 U.S.C. § 10101(12), 10 CFR Part 72.3, 

and DOE Order 5820.2A]. HLRW meeting this definition is shipped by modes other 

than rail. 

SNF and HLRW are required to be transported in casks constructed to NRC requirements. The 

casks are secured to specially constructed rail cars capable of transporting the heavy load.5

 A 

cask consist includes the cask car(s) surrounded by two buffer cars and accompanied by an 

escort car. A dedicated train is comprised of the cask consist and multiple locomotives. A 

regular or key train will include the cask consist, locomotive(s), and any number of additional 

cars potentially containing other regulated hazardous materials, various other general cargo, 

 

5

 A typical cask assembly weighs about 250,000 pounds, and a loaded cask car weighs about 394,500 pounds. A 

typical rail load weighs about 286,000 pounds. Like other cars constructed to carry heavy loads, cask cars will most 

likely use additional axles and span bolsters to distribute the weight over a larger portion of the track structure. 

Other special loads transported on the railroad include large transformers and specialized industrial equipment. 
5

and/or empty rail cars. In 2005, DOE issued a policy statement indicating “[DOE] will use 

dedicated train service (DTS) for its usual rail transport of spent nuclear fuel and high-level 

radioactive waste to the Yucca Mountain Repository site in Nevada when the repository is 

operational.” 

 

Although SNF/HLRW casks are required to be well shielded by design, some forms of radiation 

are very difficult to stop; therefore, the casks continuously emit very low levels of radiation 

throughout all phases of transportation. As a result, some unavoidable radiation exposure to 

crew, handlers, yard personnel, and the wayside population can occur whenever a shipment takes 

place. The emissions are limited to acceptable, permissible levels (a maximum of 10 millirems 

per hour (mrem/hr) at 3.3 feet (1 meter) from the surface of the package).6

 All individuals 

exposed to the radiation being emitted from the cask during transport, handling, loading, and 

unloading will receive very low doses of radiation. 

Rail cars placarded as radioactive cannot be placed next to a locomotive or an occupied 

caboose.7

 A buffer car loaded with non-radioactive material must be placed between a car 

carrying radioactive materials and a locomotive or caboose.8 

Measuring Radiation 

When discussing radiation, we measure several different phenomena. The terms “activity,” 

“exposure,” and “dose” are some of the names that are used to describe radiation and to express 

the interaction of radiation in the environment and with humans. Since this report is concerned 

with radiation exposure to railroad employees, the units of exposure and dose most relevant to 

the discussion and are defined here. 

Exposure to ionizing radiation is usually expressed in units of roentgen (R). The R unit defines 

the amount of ionization present in the air from gamma rays or x-rays. One R equals the electric 

charge of 258 microcoulombs per kilogram of air. One roentgen of gamma- or x-ray exposure 

produces approximately 1 radiation-absorbed tissue dose. 

Dose measures the effect of radiation on substances that absorb it. It measures what radiation 

does to substances, not anything specific about the radiation itself. This permits the 

measurement of different types of radiation (particles or waves) by measuring the effect they 

have on the materials. 

Rad is the acronym for radiation-absorbed dose in traditional English units. It defines the 

amount of energy from any type of ionizing radiation (e.g., alpha rays, beta rays, gamma rays, 

neutrons, etc.) deposited in any medium (e.g., water, human tissue, air). A dose of 1 rad is 

equivalent to the absorption of 100 ergs (a small but measurable amount of energy) per gram of 

absorbing medium. 

 

6

 49 CFR § 173.441 

7

 49 CFR § 174.85(b) 

8

 49 CFR § 174.85(d) 
6

Gray (Gy) is the international system (SI)9 unit of radiation dose expressed in terms of absorbed 

energy per unit mass of tissue. 

 1 Gy = 1 Joule/kilogram. 

 1 Gy = 100 rad. 

Relative Biological Effectiveness (RBE) is used to define a term known as the Quality Factor

(Q factor). Different Q factors are assigned to different types of radiation since some types are 

more dangerous to biological tissue than others, even if their “energy deposition” levels are the 

same. The value of the quality factor for each type of radiation depends on the distribution of the 

absorbed energy in a mass of tissue. 

 The Q factor is 1 for x-rays, gamma rays, and electrons. 

 The Q factor is 10 for protons and neutrons. 

 The Q factor is 20 for alpha particles. (Alpha radiation is considerably more potent than 

x-rays, beta rays, or gamma rays in causing cancer since the alpha particles that do the 

damage usually are inhaled or ingested and then incorporated in body tissue where they 

continue to emit energy.) 

The Q factor defines the relationship between rads and rems (defined below). To calculate rems 

from rads, or sieverts (defined below) from Gys, multiply by Q. The Q factor approximates what 

otherwise would be very involved computations. For example: 

 Gamma rays with the energy of 10 rad and a Q factor of 1 will produce a dose of 10 rem. 

 Alpha particles with the energy of 10 rad and a Q factor of 20 will produce a dose of 200 

rem. 

Rem is the acronym for “roentgen equivalent man.” It is the English unit of measurement of 

exposure that describes the effects of radiation specifically on human tissue. 

Sievert (Sv) is the corresponding SI unit. 

 1 Sv = 100 rem 

 1 millisievert (mSv) = 100 mrem 

Occupational exposures are characterized as doses, with limits based on the rate of exposure 

(generally mrem/hr, as well as overall accumulated exposure for a specified period). For 

example, 1250 mrem (12.5 mSv) is the maximum permissible dose for whole-body exposure per 

calendar quarter (in accordance with OSHA guidelines). 

 

9

 “SI” stands for Système International 
7

Human Exposure to Radiation 

Radiation has been naturally present in the environment since the birth of this planet. As a 

result, life has evolved in an environment with significant levels of ionizing radiation. The 

radiation comes from outer space (cosmic rays), the minerals in the ground, and within our own 

bodies since it is present in the air we breathe, the food we eat, and the water we drink. Certain 

foods grown in areas with naturally high levels of radiation in the soil contain higher levels of 

radiation than other foods. Rice and tapioca from the State of Kerala, India, and Brazil nuts, 

squash, kale, beans, cassava, and oranges are examples. 

Radiation is also found in the minerals used for or incorporated in the construction materials 

used to build our homes and other structures. Brick and stone homes have higher natural 

radiation levels than homes made of other building materials such as wood. Our Nation’s 

Capitol, which is largely constructed of granite, contains higher levels of natural radiation than 

most homes. 

The average radiation dose from exposure to natural and manmade background radiation in the 

United States is approximately 360 mrem per year. As a rule of thumb, this exposure level 

nearly doubles for each mile of elevation above sea level. Therefore, living in Denver, CO; 

Flagstaff, AZ; or other cities at high elevations increases the average background dose to 

approximately 1000 mrem per year. The increase is due to higher contributions from cosmic 

radiation at higher altitudes and terrestrial radiation sources such as radon gas. 

 Radiation Exposure from Various Sources 

Source Exposure Level

External Background Radiation 60 mrem/yr, U.S. average 

Natural Potassium–40 Radioactivity in Body 40 mrem/yr 

Air Travel–1 Roundtrip (NY–LA) 5 mrem 

Chest X-Ray–Effective Dose 10 mrem per film 

Radon in the Home ~200 mrem/yr (variable) 

Manmade (medical x-rays, etc.) ~60 mrem/yr (average) 

Risks Associated with Radiation Exposure 

The most significant risk associated with occupational exposure to ionizing radiation is the 

increased risk of cancer. The amount of that increased risk depends on three factors: the total 

dose of radiation received, the length of time over which that dose was received, and the specific 

body part or parts exposed. Genetic differences, age, and other individual personal factors are 

also thought to affect risk. 

Our understanding of the risks of radiation exposure is primarily the result of studies of Japanese 

atomic bomb survivors, the Chernobyl reactor accident survivors, radium dial painters, and 

medical patients who are exposed through selected diagnostic or therapeutic medical procedures. 
8

From these populations we know that acute (i.e., short in duration), very high radiation doses can 

increase the occurrence of certain kinds of diseases (e.g., cancer) and possibly have adverse 

genetic effects. The types of cancer associated with high-dose exposures (greater than 50,000 

mrem) include leukemia and multiple myeloma as well as breast, bladder, colon, liver, lung, 

esophagus, ovarian, and stomach cancers. The U.S. Department of Health and Human Services 

(DHHS) literature also suggests a possible association between ionizing radiation exposure and 

prostate, nasal cavity/sinuses, pharyngeal, laryngeal, and pancreatic cancers.10

Since the human body has a number of mechanisms that can repair damage caused by radiation 

and chemical carcinogens, the effects of radiation on living cells can result in different outcomes, 

including: 

 Injured or damaged cells that repair themselves with no lasting damage; 

 Injured or damaged cells that die and are replaced through normal tissue replacement 

processes (millions of cells in the body do this normally every day); or 

 Cells that incorrectly repair themselves (due to damage to the genetic code that directs the 

cell repair mechanism), resulting in a biological change. 

Although it is assumed that exposure to low levels of radiation will lead to an increased risk of 

cancer, medical studies have not yet seen these adverse health effects in people who have been 

exposed to low-level, long-term radiation doses; for example, up to 10,000 mrem above 

background for more than 2 years.11 When compared to the overall cancer rate in today’s 

society, the increased risk of cancer from normal levels of occupational radiation exposure is 

small. In the United States, the current lifetime risk of dying from all types of cancer is 

approximately 23 percent in males and 20 percent in females.12 

A very simple illustration of the theoretical increase in risk of dying from cancer due to 

occupational radiation exposure would be a person who receives a lifetime radiation dose of 10 

rem (10,000 mrem) to the entire body. This person would have a risk of dying from cancer of 

about 20%, without otherwise being exposed to radiation other than that normally present in the 

environment. With the added exposure stated above, the person’s risk of dying from cancer 

would increase to 20.4%.13

Latency and Other Sources of Damage 

The period of time between exposure to a carcinogen and the detection of cancer can be many 

years. This period is known as the “latent period.” Those cancers that may develop as a result of 

radiation exposure cannot be distinguished from cancers that have a natural origin or are a result 

 

10 U.S. NRC Fact Sheet–“Biological Effects of Radiation” 2009. 

11 The National Research Council Committee on the Biological Effects of Ionizing Radiation, Health Effects of 

Exposure to Low Levels of Ionizing Radiation, BEIR V, Washington, DC, 1990. 

12 American Cancer Society, http://www.cancer.org/docroot/CRI/content/CRI_2_6x_Lifetime_Probability_ 

of_Developing_or_Dying_From_Cancer.asp, April 2010. 

13 U.S. Nuclear Regulatory Commission, Instruction Concerning Risks from Occupational Radiation Exposure, 

Regulatory Guide 8.29, Rev. 1, NRC, Washington, DC, February 1996. 
9

of exposure to chemical carcinogens. In addition, some cancer research literature indicates that 

other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, 

and diet) significantly contribute to the incidence and severity of these same cancers. 

Although radiation may cause cancer at high doses and high-dose rates, currently there is no data 

that unambiguously establishes the occurrence of cancer following whole-body exposure to low 

doses (below about 10,000 mrem (100 mSv)) and low-dose rates. For example, people living in 

areas such as Denver, CO, where the levels of background radiation are higher than those typical 

in lower-altitude cities (near or above 1,000 mrem (10 mSv) per year)14 have not shown any 

evidence of an increase in radiation-induced cancer rates when compared to rates in other States 

and in the United States, overall. 

 

14 U.S. NRC Fact Sheet–“Biological Effects of Radiation,” 2009. 
10

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11

Radiation Safety Practice 

Radiation safety, as it is currently practiced, assumes adverse effects are possible with low-level, 

long-term exposure to radiation (i.e., less than 10,000 mrem). Radiation safety policies and 

standards have been established by international and national radiation protection organizations, 

such as the International Commission on Radiological Protection (ICRP) and the National 

Council on Radiation Protection and Measurements (NCRP), to limit potentially harmful 

radiation effects and protect the public, radiation workers, and the environment. Many of these 

policies and standards are reflected in the limits of exposure established by regulatory agencies 

such as the NRC and OSHA. All recognize the application of the “as low as reasonably 

achievable” principles in keeping exposure dose to a minimum. 

“As low as reasonably achievable,” or ALARA, is a basic radiation protection philosophy based 

on the assumption that there is no safe level of exposure to radiation. This assumption requires 

the belief that the probability for harmful biological effects increases with increased radiation 

doses, no matter how small. This assumption is under debate in the scientific community, 

reflecting the facts we know about everyday natural and manmade exposures to radiation and the 

current lack of evidence of increased risk. Much of our knowledge about health effects is 

derived from studies of populations exposed to very high doses of ionizing radiation, not from 

exposures at normal everyday levels. Application of ALARA principles means making every 

reasonable effort to maintain exposures to ionizing radiation as far below the established dose 

limits as practical, taking into account the state of technology, the economics of exposure 

reductions in relation to state of technology, the economics of exposure reductions in relation to 

the benefits to the public health and safety, etc. 

The risks associated with low-level medical, occupational, and environmental radiation exposure 

are formulated to be proportional to those observed with high-level exposure. Regulatory limits 

for the allowable exposure dose for both the public and workers are set by Federal agencies. The 

Environmental Protection Agency (EPA), OSHA, NRC, DOE and State agencies have all 

established standards to limit cancer risk. In addition, radiation dose limits have been established 

to limit other potential biological impacts on worker populations such as effects on the skin and 

lens of the eye. 

Annual Radiation Dose Limits 

Dose Agency Population covered 

5,000 mrem NRC Radiation Worker–NRC Licensee workers 

5,000 mrem OSHA Radiation Worker–non-NRC Licensee workers 

100 mrem NRC General Public–from NRC Licensee sources 

10 mrem EPA General Public–air pathway 

4 mrem EPA General Public–drinking-water pathway 

All of these regulatory limits recognize that there are many sources of radiation exposure. As 

noted above, the presence of natural “background” radiation is included in the considerations for 

the safety factors built into these limits. 
12

Protection from Exposure to Radiation 

Time, distance, and shielding are used to reduce dose due to exposure to known sources of 

ionizing radiation. 

Time. Since the dose of exposure is cumulative, the amount of time spent in proximity to a 

radiation source of a given intensity will increase the accumulated dose. If someone is exposed 

each workday to 15 mrem, their exposure will be just below the OSHA quarterly dose limit of 

1,250 mrem for whole-body exposure.15 The exposure dose of 15 mrem may be incurred 

throughout the day at about 2 mrem per hour or it may occur over a shorter period of higher 

exposure with offsetting periods of lower or no exposure. 

Distance. Distance is another important concept, but a little more involved. Since radiation is a 

physical phenomenon, the intensity of radiation energy decreases at a known rate as the distance 

from the source increases. The rate of this decrease follows what is known as the “inverse 

square rule.” 

The inverse square rule describes how physical phenomena spread influence equally in all 

directions without a limit to range—a geometrically spherical spread. The intensity of the 

phenomenon at any given radius (r) is the source strength divided by the area of the sphere. 

This means that you can predict the intensity of the energy contained in the particles or rays of 

radiation at a different location if you have determined the amount of radiation at a particular 

distance from the source. If you measure the intensity at 1 meter and it is 200 mrem, then it will 

be one fourth as much at 2 meters or 50 mrem, following the inverse square rule. 

Shielding. Shielding is a barrier of some kind between the source of radiation and its 

surroundings. Alpha and beta particle radiation is easily stopped by materials with low 

thicknesses and densities. As an example, a sheet of ordinary paper will stop almost all alpha 

radiation and a thin sheet of plastic or metal will stop beta particles. More energetic photon-type 

radiation—gamma rays and x-rays—require thicker, more dense shielding. 

The effectiveness of a shielding material, in general, increases with its electron density. Highmass density materials like lead have high electron densities. A sheet of lead is more effective 

than a sheet of steel of the same thickness, since lead has a higher electron density than steel. 

Another important property of shielding is that it exponentially reduces the intensity of radiation 

depending on its thickness. 

 A lead shield that is 1 cm (0.4 in) thick reduces the level of gamma radiation by 50% (for 

example, from 200 mrem to 100 mrem). 

 To get the same level of shielding from steel, the sheet would have to be 2.5 times as 

thick. 

 

15 OSHA has three different permissible limits for ionizing radiation based on the body part exposed. See the 

reproduction of the OSHA Table G-18 in the discussion of OSHA on page 18. 
13

Both of the examples above are the half-value layer thicknesses of lead and steel, respectively. 

Half-value layer thickness means that each time you add another layer of a particular material of 

the same thickness, the level of radiation getting through is reduced by half of the incident level 

at the other side of the shielding. That means: 

 A lead shield that is 5 cm (2 in) would reduce the level from 200 mrem to 6.25 mrem 

(5 half-value layers of lead). 

 It would take 5 inches of steel to provide the same level of shielding (5 half-value layers 

of steel). 

It is also true that shielding of the same material will provide the same level of protection for a 

given type of radiation regardless of the source of the radiation. Therefore, x-rays, whether they 

are from an x-ray machine or from a radioactive material that emits x-rays during radioactive 

decay, can be well-shielded by lead. Lead and other high-mass density materials are also good 

shielding materials for gamma radiation. Gamma radiation is the same type of radiation as xrays (electromagnetic radiation), but it may differ significantly in energy from some x-rays. 

One type of radiation that is not sufficiently shielded by typical high-mass density materials is 

neutron radiation. Neutrons are electrically neutral particles with significant energy and can pass 

fairly easily through many materials since they have no charge and do not interact with the 

electrons of materials through which they pass. In fact, low-mass materials are better for neutron 

shielding since interaction with the nucleus of their atoms are more likely to result in the transfer 

of the neutron energy to the nucleus. Hydrogen is the lowest-mass atom, so materials with high 

hydrogen content, such as water and plastics or other organic materials with high hydrogen 

content, make good neutron shielding. Concrete is another material frequently used for neutron 

shielding since the chemistry of concrete incorporates water into the finished material. 
14

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15

Regulatory Regimes and Exposure Limits 

There are several sets of regulations that are pertinent with respect to occupational exposures to 

radiation that may be encountered by railroad employees. While a number of the regulations 

have been mentioned already, this section provides an overview. 

 DOE regulates exposure to ionizing radiation for employees at DOE facilities, including 

both Federal and contractor employees. This includes the regulation of all aspects of 

DOE shipments exclusive of the regulation of cask certificates, which are regulated by 

NRC. 

 The Department of Defense (DOD) is responsible for worker exposures to ionizing 

radiation in DOD facilities and operations. 

 NRC regulates worker and public exposure to ionizing radiation from specific materials 

for which NRC issues licenses. This includes transportation of those materials in the 

packages that they regulate. 

 OSHA regulates worker exposure to ionizing radiation in many workplaces. OSHA 

standards cover worker exposures from all radiation sources except those identified 

above. These sources include x-ray equipment, accelerators, accelerator-produced 

materials, electron microscopes, and naturally occurring radioactive materials (NORM). 

 DOT, through enforcement of limits on package emissions16 and, for rail transportation, 

placement within trains,17 limits worker and public exposures to ionizing radiation during 

transportation. 

Of these regulations, the ones that are most relevant to this report and the exposures discussed 

here are those of NRC (in conjunction with DOT) and OSHA. 

Nuclear Regulatory Commission Standards and Limits 

As mentioned in the discussion of SNF/HLRW, the NRC establishes permitted levels of 

emissions from the casks used for transporting these materials. The NRC’s regulations for 

transporting radioactive materials are found in 10 CFR Part 71. These limits are codified for 

enforcement by DOT under 49 CFR § 173.441. Section 71.47 defines external radiation 

standards for all packages: 

§ 71.47 External radiation standards for all packages. 

(a) Except as provided in paragraph (b) of this section, each package of 

radioactive materials offered for transportation must be designed and prepared for 

shipment so that under conditions normally incident to transportation the radiation 

level does not exceed 2 mSv/h (200 mrem/h) at any point on the external surface 

of the package, and the transport index does not exceed 10. 

(b) A package that exceeds the radiation level limits specified in paragraph (a) of 

this section must be transported by exclusive use shipment only, and the radiation 

levels for such shipment must not exceed the following during transportation: 

 

16 49 CFR § 173.441 

17 49 CFR § 174.85 
16

(1) 2 mSv/h (200 mrem/h) on the external surface of the package, unless the 

following conditions are met, in which case the limit is 10 mSv/h (1000 mrem/h): 

(i) The shipment is made in a closed transport vehicle; 

(ii) The package is secured within the vehicle so that its position remains fixed 

during transportation; and 

(iii) There are no loading or unloading operations between the beginning and end 

of the transportation; 

(2) 2 mSv/h (200 mrem/h) at any point on the outer surface of the vehicle, 

including the top and underside of the vehicle; or in the case of a flat-bed style 

vehicle, at any point on the vertical planes projected from the outer edges of the 

vehicle, on the upper surface of the load or enclosure, if used, and on the lower 

external surface of the vehicle; and 

(3) 0.1 mSv/h (10 mrem/h) at any point 2 meters (80 in) from the outer lateral 

surfaces of the vehicle (excluding the top and underside of the vehicle); or in the 

case of a flat-bed style vehicle, at any point 2 meters (6.6 feet) from the vertical 

planes projected by the outer edges of the vehicle (excluding the top and 

underside of the vehicle); and 

(4) 0.02 mSv/h (2 mrem/h) in any normally occupied space, except that this 

provision does not apply to private carriers, if exposed personnel under their 

control wear radiation dosimetry devices in conformance with 10 CFR 20.1502. 

(c) For shipments made under the provisions of paragraph (b) of this section, the 

shipper shall provide specific written instructions to the carrier for maintenance of 

the exclusive use shipment controls. The instructions must be included with the 

shipping paper information. 

(d) The written instructions required for exclusive use shipments must be 

sufficient so that, when followed, they will cause the carrier to avoid actions that 

will unnecessarily delay delivery or unnecessarily result in increased radiation 

levels or radiation exposures to transport workers or members of the general 

public. 

 

Occupational Safety and Health Administration Standards and Limits 

OSHA regulates worker exposure to ionizing radiation under the authority granted by the 

Occupational Safety and Health Act of 1970 (the Act) (29 U.S.C. 651, et seq.). OSHA standards 

cover worker exposures from all other radiation sources not identified in the regulations of DOD, 

DOE, or NRC; including x-ray equipment, accelerators, accelerator-produced materials, electron 

microscopes, and NORM. OSHA continues to work with NRC, DOE, DOD, and the EPA on 

advances in the research and data collection dealing with worker exposure and Federal policies 

addressing this important issue. OSHA also continues its involvement with the Interagency 

Steering Committee on Radiation Standards in an effort to coordinate any future activity. 

OSHA has published its standards in 29 CFR § 1910.1096 under the title, “Ionizing Radiation.” 

The standard was originally issued and subsequently amended as described in the following 

caption: [39 FR 23502, June 27, 1974, as amended at 43 FR 49746, Oct. 24, 1978; 43 FR 51759, 

Nov. 7, 1978; 49 FR 18295, Apr. 30, 1984; 58 FR 35309, June 30, 1993; 61 FR 5507, Feb. 13, 

1996; 61 FR 31427, June 20, 1996]. 
17

Under the OSHA standard, some of the definitions previously discussed for dose and the units 

used to describe exposure are refined as follows: 

1910.1096(a)(5) - Dose means the quantity of ionizing radiation absorbed, per 

unit of mass, by the body or by any portion of the body. When the provisions in 

this section specify a dose during a period of time, the dose is the total quantity of 

radiation absorbed, per unit of mass, by the body or by any portion of the body 

during such period of time. Several different units of dose are in current use. 

Definitions of units used in this section are set forth in paragraphs (a)(6) and (7) 

of this section. 

1910.1096(a)(6) - Rad means a measure of the dose of any ionizing radiation to 

body tissues in terms of the energy absorbed per unit of mass of the tissue. One 

rad is the dose corresponding to the absorption of 100 ergs per gram of tissue (1 

millirad (mrad)=0.001 rad). 

1910.1096(a)(7) - Rem means a measure of the dose of any ionizing radiation to 

body tissue in terms of its estimated biological effect relative to a dose of 1 

roentgen (r) of X-rays (1 millirem (mrem)=0.001 rem). The relation of the rem to 

other dose units depends upon the biological effect under consideration and upon 

the conditions for irradiation. Each of the following is considered to be 

equivalent to a dose of 1 rem: 

1910.1096(a)(7)(i) - A dose of 1 roentgen due to X- or gamma radiation. 

1910.1096(a)(7)(ii) - A dose of 1 rad due to X-, gamma, or beta radiation. 

1910.1096(a)(7)(iii) - A dose of 0.1 rad due to neutrons or high energy protons. 

1910.1096(a)(7)(iv) - A dose of 0.05 rad due to particles heavier than protons and 

with sufficient energy to reach the lens of the eye. 

In Section (b) of the OSHA standard, the exposure limits are established as follows: 

(b) Exposure of individuals to radiation in restricted areas. 

(1) Except as provided in paragraph (b)(2) of this section, no employer shall 

possess, use, or transfer sources of ionizing radiation in such a manner as to cause 

any individual in a restricted area to receive in any period of one calendar quarter 

from sources in the employer’s possession or control a dose in excess of the 

limits specified in Table G-18: 

TABLE G-18 Rems per calendar quarter

Whole body: Head and trunk; active 

blood-forming organs; lens of eyes; or 

gonads 

1-1/4 (1,250 mrem) 

Hands and forearms; feet and ankles 18-3/4 (18,750 mrem) 

Skin of whole body 7-1/2 (7,500 mrem) 
18

(2) An employer may permit an individual in a restricted area to receive doses to 

the whole body greater than those permitted under subparagraph (1) of this 

paragraph, so long as: 

(i) During any calendar quarter the dose to the whole body shall not exceed 3 

rems; and 

(ii) The dose to the whole body, when added to the accumulated occupational 

dose to the whole body, shall not exceed 5 (N-18) rems, where “N” equals the 

individual’s age in years at his last birthday; and 

(iii) The employer maintains adequate past and current exposure records which 

show that the addition of such a dose will not cause the individual to exceed the 

amount authorized in this subparagraph. As used in this subparagraph Dose to the 

whole body shall be deemed to include any dose to the whole body, gonad, active 

blood forming organs, head and trunk, or lens of the eye. 

(3) No employer shall permit any employee who is under 18 years of age to 

receive in any period of one calendar quarter a dose in excess of 10 percent of the 

limits specified in Table G-18. 
19

Previous Research and Predicted Levels of Exposure 

As mentioned earlier, the Dedicated Train Study, or DTS, conducted under a prior congressional 

mandate,18 examined the safety and integrity of the packaging and shipment of SNF/HLRW. 

Contributing to the information upon which the DTS was based, the John A. Volpe National 

Transportation Systems Center (Volpe), Accident Prevention Division (DTS-73) (Cambridge, 

MA), provided a technical analysis and report comparing the relative safety of regular trains 

versus dedicated trains used for shipping these materials.19

Part of the analysis contained in the DTS was an estimate of the potential radiation exposures to 

various populations including different groups of railroad employees involved in these 

shipments. Several sets of assumptions were made in estimating exposures, and those most 

relevant to this report are quoted below. Please note that the term “incident free” is used in the 

Volpe report and DTS to distinguish the risks and levels of exposure arising simply from 

activities associated with transportation of the materials, rather than those associated with 

accidents that might occur. The “incident-free” exposure estimates are relevant to this report’s 

focus. 

For those unfamiliar with a dedicated train, Figure 1, below, illustrates a typical consist. 

Source Strength 

The Volpe report estimated the intensity of the radiation emitted from the packages by using the 

maximum allowable limits established by the NRC. On page 20, the report states: 

Packaging, transport and disposal of radioactive materials by all modes of 

transportation is regulated in the United States by the NRC and the DOT. 

Regulations promulgated by the NRC are contained in Title 10 of the Code of 

Federal Regulations (10 CFR 71-73); regulations promulgated by the DOT are 

primarily contained in Title 49 (49 CFR 171-178). These regulations establish 

maximum permissible package dose rates and maximum permissible dose rates to 

vehicle crew members. Characteristics of radioactive material that affect incident-

 

18 Section 15 of the Hazardous Materials Transportation Uniform Safety Act of 1990 (Pub. L. No. 101-615)—

amended Section 116 of the Hazardous Materials Transportation Act (49 U.S.C. App. 1813) 

19 “Comparative Safety of the Transport of High-Level Radioactive Materials on Dedicated, Key and Regular 

Trains” Volpe Report Number DOT-VNTSC-FRA-05-06; FRA Report Number FRA/ORD-05/03. 

Figure 1: Dedicated Train Consist 
20

free transportation are the package dose rate and the fractions of gamma and 

neutron radiation. The package dose rate is expressed as a transportation index 

(TI) for certain package types. TI is defined as the highest radiation dose rate in 

millirem per hour (mrem/hr) from all penetrating radiation at 3.3 feet (1 meter 

(m)) from any accessible external surface of the package, rounded to the highest 

tenth (49 CFR 173.403). For the purposes of this analysis, it was conservatively 

assumed that the dose rate is the regulatory limit of 10 mrem/hr at 3.3 feet (1 

meter). The estimated dose rate for the MPC cask selected for this analysis is 

below this regulatory limit. (Emphasis added.) 

The next relevant section was the identification and characterization of the populations that 

would be exposed to radiation during the transportation SNF and HLRW by rail. On page 24, 

the groups of railroad employees that were identified included: 

 Train Crews: Train crews are estimated at two per train for the dedicated, 

regular, and key trains. 

 Shipment Escorts: Four escorts per train are assumed for dedicated, regular, and 

key trains. 

 Inspectors/Classification Yard Workers: Railroad employees that classify or 

inspect the rail casks cars during stops are likely to receive close proximity 

exposures. Functions performed at stops include marshalling of cars, arrival and 

departure train inspections, and repair of damaged railcars. A determination of 

exact numbers of close-in rail yard workers was not established. Instead, doses 

for this population were estimated based on the total person-hour/meter estimate 

used by RADTRAN [DOE, 1986]. 

 Other Rail Yard Workers: An average of 125 workers within a 0.2 mi2

 (0.5 

km2

) area at each yard is assumed based on estimates provided by consulted 

railroads. This gives a yard worker population density of 625 workers per mi2

(250 workers per km2). 

Another key issue discussed was the proximity of the various groups to the radioactive sources—

recognizing one of the key means of reducing exposure—distance. Beginning on page 24, the 

report says: 

2.1.5 Distances from the Source

The distance from the source is a determining factor in the amount of radiation 

dose members of a population group receive. Distance is important because the 

radiation level varies with the inverse square of the distance from the cask. The 

various impacted populations are at different distances from the source. 

Train Crew. Train crew distances from the cask vary depending on the shipment 

service selected. The cask car(s) were assumed to be buffered front and rear. A 

49.2 ft (15-m) car length and 6.6 ft (2 m) between cars was assumed. For regular 

and key train service, it was assumed that the cask car was car number 35 in a 70 

car train. For dedicated service, it was assumed that the train consisted of two 

locomotives (with crew in first unit), buffer car, cask car, buffer car, and escort 
21

car (see Figure 6). Crew distances were thus 2,140 ft (652.3 m) and 300 ft (91.3 

m) for regular/key and dedicated service, respectively. 

Shipment Escorts For all service cases it was assumed that the escort distance 

from the cask was 96 ft (29.3 m). The cask was assumed to be buffered front and 

rear, with escorts in a car following the rear buffer car. Although the position of 

the escort railcar could differ for regular and key train service, placement used for 

this analysis results in the most conservative estimate. 

The issue of time of exposure relative to the railroad employees was discussed. The assumptions 

made begin on page 26 with this statement. 

Exposure time is a determining factor in the amount of radiation members of a 

population group receive. In determining the total exposure durations of 

populations, time spent near both moving and standing trains is considered. Train 

operational restrictions such as train speed and run through operations impact 

exposure time both during stops and when en-route. 

On page 27, operating crew and escort exposures are described. 

For Moving Train. Exposure time for moving trains is dependent on the train 

speed and route length ... Speeds of both 35 mph (56.3 km/hr) and 50 mph (80.4 

km/hr) were used for this analysis. For crews and escorts, transit time was 

calculated for each route by multiplying the average speed by the route length. 

(Emphasis added.)

For Standing Trains. Two types of stops were assumed for each route: yard 

stops (classification, switching, and inspection) and non-yard or siding stops 

(interchange and crew change). Each type has a different stop duration. Stop times 

for regular and dedicated trains differ since handling, inspections, routes, crew 

changes, and many other variables affect the time. ... In general, regular and key 

trains stop in every yard; dedicated trains stop for crew changes (driven by hoursof-service limits) and when entering territory of a different railroad and changing 

locomotives (about every 350 miles (563 km)). Trains also could be stopped for 

inspections (the assumption for this analysis is that these inspections are done at 

the nearest siding/yard stop). 

On page 28, crew and escort exposures are discussed further: 

Crew and escort in-transit exposure was calculated as a stop with a duration equal 

to the total travel time for the trip. Actual stop time for the crew is equal to the 

total travel time, plus two hours for each yard stop, excluding origin and 

destination (O-D), plus non-classification [sic] stop time. Escort stop time is equal 

to the total travel time, plus the full yard entry times including O-D (it is assumed 

escorts never leave the shipment), plus non-classification [sic] (interchange, crew 

change, refueling, inspection) stop times. Note that the number of nonclassification stops for regular and key trains are fewer than for dedicated trains 

because some crew changes are assumed to occur in conjunction with 

classification stops. 
22

The final element of the three protective measures that reduce exposures is shielding. The Volpe 

report addresses shielding on page 28 as follows: 

2.1.7 Shielding Factors

The amount of shielding between the source and the affected population impacts 

the received dose rate. 

Shielding factors are then summarized in a table. The excerpt below shows only the shielding 

factors affecting railroad workers. 

Table 10. Shielding Factor (Attenuation) 

 Receptor 

 Shielding 

Population Factor Construction Type 

Crew 0.5 Reflects gamma radiation attenuation by 

 locomotives 

Escorts 1.0 No shielding 

Inspectors/Handlers 1.0 No shielding 

General Yard 0.1 Reflects gamma radiation attenuation by 

and Workers other structures in the rail yard railcars 

After considering all of these factors, the Volpe study estimated the potential exposures of the 

different populations exposed during the transportation of the SNF/HLRW. These estimates 

were made using a software application called RADTRAN 5, which was developed by Sandia 

National Laboratories. RADTRAN 5 constructs simulations to estimate population effects of 

these kinds of shipments. This radiation report will look at the estimates for the railroad 

populations and begin with the explanation of the results on page 29 of the Volpe study. 

2.2 INCIDENT-FREE RESULTS

The following section presents the radiological consequences of “incident-free” 

transportation of HLRW and SNF by the regular train, key train and dedicated 

train service modes for both the 35 mph (56.3 km/hr) and 50 mph (80.4 km/hr) 

speeds. The results are presented by route, service/speed, population type, and intransit vs. stops. The intent of the incident-free analysis was to provide a general 

estimate of the differences between the alternate service modes and speeds. 

Simulations of the alternatives were conducted comparing service types for the 

same sets of routes. The results of these estimates are included as an example of 

the likely differences in exposure due to changes in service characteristics. All 

incident free radiological impact results are given for a single shipment, i.e., a 

single movement of a single cask. 

In general, these results show that dedicated trains expose populations to a lesser 

radiological dose than regular and key trains at all speeds, and that stop time risk 

dominates total exposure for regular and key trains. 
23

The dose estimates from the DTS that are most pertinent to this report are those for dedicated 

trains. It is important to note that the DOE has stated its intention to exclusively employ 

dedicated trains in its movements of laden casks. Those estimates are compiled below. Since it 

is difficult to predict with certainty the numbers of different groups of railroad workers who may 

be present, with the exception of the train crew and escorts, all values from the DTS estimates 

will assume a population of two employees in each group. The exposures will be translated from 

person-rems to exposure doses of mrem for comparison with OSHA and NRC regulatory limits. 

Train Crew and Escorts 

“For train crews, dedicated train doses are higher than for the regular and key 

trains (assuming no special shielding provisions), primarily because of the closer 

proximity of the crew to the cask in the dedicated train. In-transit results are also 

speed dependent, with higher train speeds generating lower doses. Train crews 

could receive between a 1.17×10-05 and 1.62×10-03 person-rem dose per 

shipment.”20

Since most train crews are made up of an engineer and a conductor, these values (0.0000117 to 

0.00162 person-rem) translate to exposure doses that could range from 0.00585 mrem to 0.81 

mrem per person per trip. (Note: It is assumed that the lower exposures reflected by the model 

used in the DTS are due to the shielding and distance from the source provided by the 

locomotive(s) used in the consist, as well as the shorter periods of exposure than the escorts, due 

to compliance with the hours of service law.) 

 

“For shipment escorts, dedicated train case doses are lower than regular and key 

train cases for both speed scenarios because of the shorter stop durations. Stop 

doses are higher than the in-transit doses for the regular and key train cases. 

Escorts could receive between a 0.108 and 0.041 person-rem dose per 

shipment.”21

The number of escorts accompanying these shipments is not public information for security 

reasons. For the purposes of this report, we will use five persons to provide a basis for 

comparison. These values (0.108 to 0.041 person-rem) translate to exposure doses that could 

range from 22 mrem to 8 mrem per person per trip. 

 

Car Inspectors and Close Proximity Yard Workers 

“Car inspectors/classification workers could receive stop doses between 0.0056 

and 0.0613 person-rem per shipment. Since the exposures to this population group 

are for stops only (no in-transit), results are not speed dependent, but are driven 

 

20 Ibid., page 38 

21 Ibid., page 38 

The term person-rem is used in this study to characterize the collective 

exposure dose for a particular group of people. The unit represents the product 

of the average dose per person times the number of people exposed (e.g., an 

exposure of 5 rem to each of 1,000 persons = 5,000 person-rem). 
24

by the number and duration of stops, which are route specific. In all cases, doses 

for dedicated trains are less than for regular and key trains.”22

For two workers in this class, these values (0.0056 to 0.0613 person-rem) translate to exposure 

doses that could range from 2.8 mrem to 30.65 mrem per person per trip. 

Rail Yard Workers 

“Rail yard workers (other than classification workers) could receive stop doses 

between 2.62×10-03 and 6.09×10-03 person-rem per shipment. Since the exposure 

for this population is for stops only (excludes intransit), results are driven by the 

number and duration of stops that are route specific. In all cases, doses for the 

dedicated train cases are less than the regular and key train cases.”23

For two workers in this class, these values (0.00262 to 0.00609 person-rem) translate to exposure 

doses that could range from 1.31 mrem to 3.05 mrem per person per trip. 

Conclusions about the DTS Exposure Estimates 

DTS attempted to estimate the potential exposures to ionizing radiation from HLRW/SNF 

shipments among the various populations, including the different classes of railroad workers. 

Among those railroad workers, the highest potential exposures, due to proximity and time of 

exposure, were among the escorts. 

In perspective, if escorts performed this duty over the shortest route studied, the Humboldt 

Nuclear Power Plant in California to Yucca Mountain—a distance of 1,090 miles—with an 

average speed of 33 mph, including 21⁄2-hour stops for train crew changes every 11 hours and 

1,090 miles traveled at 35 mph, the exposure would occur over 32.25 hours. If this same crew of 

escorts did this once a week for 50 weeks a year, their annual dose would be 1,100 mrem, just 

under one quarter of the OSHA permitted annual whole body dose. 

Keep in mind that this is a theoretical dose estimate based on the permitted emissions from the 

casks, a maximum of 10 mrem/hr at 3.3 feet (1 meter) from the surface of the package (49 CFR 

§ 173.441). 

 

22 Ibid., page 39 

23 Ibid., page 40
25

Real-World Exposures 

U.S. Railroad Data–Norfolk Southern Railway 

Beginning in May of 2005, NS initiated occupational exposure monitoring of employees 

involved in shipments of SNF for a U.S. Government client. These shipments involved 

dedicated trains made up in accordance with Association of American Railroads (AAR) 

Recommended Practice (AAR Circular No. OT-55-J, “Recommended Railroad Operating 

Practices for Transportation of Hazardous Materials”) as detailed in Appendix A to this report. 

FRA requested and received the NS SNF shipment radiation monitoring data. This data included 

train information and laboratory reports showing the results of NS’s personal dosimetry 

monitoring (anonymous) and contractor cask monitoring reports (when available). 

The data covers the only trains NS operated carrying SNF/HLNW; the intent was to monitor all 

such trains. Eight trains were monitored from May 2005 to October 2009. 

All employees actively involved with the shipment were monitored for the full duration of their 

exposure. The data covered 176 individual employee measurements. NS employees monitored 

included locomotive engineers, conductors, transportation supervisors, environmental personnel, 

police, and various other NS personnel involved in the actual movements. All involved 

employees were provided video training before participation, and were provided test results in 

writing after the trips were completed. 

Monitoring was done by means of thermoluminescent dosimeters (TLD)24 which detect gamma, 

x-ray, beta, and neutron radiation with a detection limit of 10 mrem. The shortest period of time 

an employee would have been exposed was 6 hours. 

All of the 176 employee exposure results were less than the detection limits of the dosimeters, 

with one exception—an employee (conductor) whose exposure was 13 mrem. The source of the 

exposure was not identified, so it could have been from the shipment or another external source. 

In the context of relevant occupational exposure limits, this exposure could occur almost 400 

times in a year (or about twice a workday) and still be below the OSHA limit for annual 

exposure. 

In addition to employee exposure measurements, the level of radiation at the surface of and 

surrounding the package (cask) were measured to determine compliance with the NRC/DOT 

limits for this type of shipment. The highest source measurements on the consist for these 

shipments were taken in contact with the surface of the cask; the measurements ranged from 0.60 

mrem/hr to 0.45 mrem/hr for combined gamma and neutron radiation. All of these 

measurements were found on the bottom surface of the cask, a location where it is unlikely any 

member of the crew, or other railroad workers, would be occupying. 

 

24 Please see Appendix B for an explanation of the TLD technology. 
26

The highest measurement at 1 meter from the surface of the cask for combined neutron/gamma 

radiation anywhere around the cask was 0.10 mrem/hr. Figure 2, above, shows a set of typical 

measurements and their locations around the cask. The monitoring results indicate very low 

potential for exposures in excess of the detection limit of the monitoring devices to NS 

employees involved in the shipments as measured since they began in 2005. The emission level 

measurements are significantly lower than those permitted by the applicable regulations and, as a 

result, the employee exposures were well below the permitted limits. 

The actual dose measurements are consistent with the levels predicted by the DTS for most of 

the classes of railroad employees, and significantly lower than those predicted for the escort 

personnel. 

Figure 2: Cask Emission Measurements for NS 10/23/2009 
27

The French Study 

In 2004, the French Railway, Société Nationale des Chemins de fer Francais (SNCF), requested 

the assistance of the Institute for Radiological Protection and Nuclear Safety (IRSN)25 to 

characterize the exposures of rail workers to ionizing radiation.26 The units used to describe 

exposures characterized in the study were μSv/hr (micro Sieverts per hour). The following 

conversion was used, and all the numerical data contained in the IRSN study were converted to 

mrem for ease of comparison with other data in this report. 

In France, radioactive fuel and wastes are transported mainly by train and on a routine basis. 

The genesis of the study was recognition of this and the fact that employees are exposed. Rather 

than paraphrase, the essence of the study approach is summarized in the following quoted 

passages: 

“Radioactive fuel and wastes are frequently transported for storage and/or 

reprocessing purposes. The main part of this transport is generally done by train. 

Before, during and after the journey, operators and drivers, who work directly in 

contact with and in the vicinity of the wagons, are exposed to external irradiations 

due to the radioactive materials that are confined inside the containers.” 

SNCF (French Railways) Directive RH 0824 relating to the prevention of 

accidents and protection against the risk of exposure to radiation and 

contamination during the carriage of radioactive goods by rail requires all 

shipments of this type of material to be covered by a Radiological Protection 

Programme (RPP). As part of this programme, SNCF is required to make an 

assessment of the external exposure to ionising radiation to which employees may 

be subjected. 

SNCF has asked the External Dosimetry Department of the Institute for 

Radiological Protection and Safety (IRSN) to carry out the necessary 

measurements in order to establish the values of ambient dose equivalents H(10) 

in the vicinity of shipments of radioactive materials, for convoys of nine different 

types, that are considered to be representative of all types of possible transports, 

involving photon27 or mixed neutron–photon fields. 

 

25 The IRSN is an agency of the French Government under the joint authority of the Defense Minister, the 

Environmental Minister, the Industry Minister, and the Health and Research Minister, with expert staff that perform 

investigations and studies in the fields of nuclear safety, protection against ionizing radiation, protection and control 

of nuclear material, and protection against accidents associated with these areas. 26 “Workplace Characterisation in Case of Rail Transport of Radioactive Materials” L. Donadille, C. Itie, T. 

Lahaye, H. Muller, F. Trompier and J. F. Bottollier-Depois Institute for Radiological Protection and Nuclear Safety 

(IRSN), BP 17, F-92262 Fontenay-aux-Roses, France, Radiation Protection Dosimetry (2007), Vol. 125, No. 1-4, 

pp. 369-375 

27 Photon radiation measured consisted of x-rays and gamma rays. 

1 Sv = 100 Rem 

1 mSv = 0.1 Rem = 100 mrem 

= 0.1 mrem 
28

The measurement campaign had started in May 2004 and four types of radioactive 

convoys had already been investigated. By using survey meters and 

spectrometers, the study consisted in measuring the external exposure at different 

stages of the work that was done beside the wagons (e.g. coupling/decoupling two 

wagons, checking the brakes, etc.) and inside the locomotive (driving). For each 

one of these tasks, the exposure was estimated in terms of H*(10)28 by summing 

the dose all along the different phases carried out by the operator. In addition, a 

dosimetric characterisation of each convoy was made by performing 

measurements along the wagons and spectrometric information about the photon 

and/or neutron fields were collected. This study provides helpful data to predict 

the dose that the operators are liable to integrate over long periods, typically 

1 y.”29

A variety of instruments were used that were more sophisticated than the TLDs used in NS’s 

series of measurements described above. They were capable of measuring exposure over a wide 

range of intensities, from well below the 10 mrem lower level of detection of the TLDs to several 

thousand mrem. These instruments were used to ensure a variety of different types of emissions, 

including photon (x-rays, gamma rays) and neutron, were properly characterized. Where 

different types of instruments with different sensitivities were used for measuring the same 

emission types, the appropriate correction factors were applied to the data to ensure consistency 

in reporting. 

In order to ensure that the measurements would not be affected by other shipments of radioactive 

materials, a loaded consist was placed at an isolated location. In addition, the activities of each 

of the groups of railroad workers who would be involved with the shipments were simulated and 

timed to ensure that projections of annual doses would be realistically characterized. 

The exposures were then measured at locations that represented either the highest potential for 

exposure or where an employee would spend the most time, e.g., the cab of the locomotive. In 

the case of employees whose jobs required movement along the consist, which would result in 

exposures to different levels, the method used measured exposures at the locations, reproducing 

the exposure times at those locations and the movements of the employees. 

 

28 The instruments used in this study were designed to measure H*(10)–the dose (rate) at some place in air (not at 

the body of a person). This was done to estimate the dose a person would receive if he or she would be at that same 

place for some time. This method of measurement was created to account for changes to the radiation field caused 

by the body of the person who finally went to that place without requiring a person to be actually exposed to the 

radiation field. 

29 “Workplace Characterisation in Case of Rail Transport of Radioactive Materials” L. Donadille, C. Itie, T. 

Lahaye, H. Muller, F. Trompier and J. F. Bottollier-Depois Institute for Radiological Protection and Nuclear Safety 

(IRSN), BP 17, F-92262 Fontenay-aux-Roses, France, Radiation Protection Dosimetry (2007), Vol. 125, No. 1-4, 

p. 369
29

The mapping of the intensities of radiation emissions in contact with each car in the consist are 

shown above in Figure 3. It shows a summary of the data reported in the study, with the values 

converted from μSv/hr to mrem/hr for ease of comparison with the values used in the United 

States. 

For simplicity, the total dose measured is reproduced in the table below for comparison with 

TLD exposures reported in the NS study. 

The final part of the study used the radiation exposure data and the time of exposure-by-task data 

to estimate an overall exposure dose. The study estimated that an employee who would conduct 

each of the tasks (with the exception of driving) 100 times would have an exposure of about 

25 mrem. The driving task was estimated based on the hours exposed (see Table 1 below). 

Figure 3: SNCF/IRSN Surface Measurements 
30

Table 1: SNCF/IRSN Data Table 

Task Exposure time(s) 

Total Dose* 

mrem 

Coupling engine to wagon 40 0.001 

Uncoupling engine from wagon 40 0.001 

Testing brakes 300 0.065 

Placing and removing rear warning lamps 70 0.015 

Checking train status 240 0.051 

Recording train details 360 0.077 

Dispatching the train 70 0.031 

Total 1,120 0.241 

 

Driving (mrem/hr)** 0.02 

 * Exposure, expressed in terms of H*(10), associated with the essential operational tasks. 

**The results for the driving task are expressed in mrem/hr since the driving time cannot be easily estimated. 

The results reported in this study indicate that, even with the different methods used to obtain 

exposure determinations, the exposures measured were of a similar order of magnitude as those 

predicted by the DTS and found in the NS studies. 

The German Study 

In July of 2003, a research study30 was the basis for a presentation at the International 

Conference on the Safety of Transport of Radioactive Material sponsored by the International 

Atomic Energy Agency (IAEA). The paper summarized the principal findings and conclusions 

of a survey of radiation exposures incurred by workers and the public from the normal transport 

of radioactive material in Germany.31

The survey covered all major categories of radioactive materials, the large majority of these 

packages contained only relatively small quantities of radiopharmaceuticals, research and 

industrial sources, and other radioactive commodities. The study excluded consumer goods such 

as smoke detectors. The study covered approximately 750,000 radioactive material packages 

shipped annually over an 8-year period in Germany by all transportation modes, i.e., by road, 

rail, air, and sea. Large quantity shipments of radioactive materials, such as HLRW or SNF, 

accounted only for a small proportion of the total volume of radioactive material shipments 

within Germany. 

 

30 “Assessment of the Radiation Exposures associated with the Transport of Radioactive Material in Germany,” 

G. Schwartz and F. Lange, Gesellschaft fur Anlagen und Realktorsicherheit (GRS) mbH, Schwertnergasse 1, 50667 

Koln, Germany – Proc. IAEA Conference, July 7–11, 2003, Vienna, page 97. 

31 GRS mbH is a nonprofit, scientific and technical expert and research organization. It is Germany’s leading 

expert institution in the area of nuclear safety and waste management. 
31

The table below is an excerpt from the results of the survey and assessment in terms of 

occupational radiation exposures arising from the normal transport32 of radioactive material in 

Germany. The transport-related doses cover a range of transport activities and cover fuel cycle 

and non-fuel cycle radioactive material shipments and their predominant mode of transport 

including the following: 

 Unirradiated nuclear fuel cycle material, e.g., uranium concentrate, uranium hexafluoride, 

UO2-powder/pellets, fuel elements, and pins, etc. 

 Irradiated nuclear fuel cycle material includes SNF, vitrified HLRW, irradiated fuel pins, 

etc., and large quantity radiation sources. 

 Non-nuclear radioactive waste, e.g., medical and research waste. 

 Supply and distribution of medical, research, and industrial isotopes. 

 Radiography sources. 

Table 2: Occupational Radiation Exposures Arising from Normal Transport 

Material Category/Transport Activity Transport Mode 

Maximum 

Effective Dose Per 

Worker (mrem/a)* 

Unirradiated fuel cycle material, e.g., U3O8, UF6, UO2-

powder/pellets, fuel pins & fuel assemblies, radiation 

sources 

Road/Rail < 100 

Unirradiated/irradiated nuclear fuel cycle material and large 

quantity radiation sources, e.g., activated/contaminated 

equipment and components, radioactive waste, spent nuclear 

fuel, high level radioactive waste etc. 

Road 100–300 

Rail < 100 

* This unit is mrem per annum (per year)–an annual dose estimate.

In discussing the findings of the report the authors stated that: 

The exposure data presented in Figure 1 and Table 1 for the recent years indicate 

that the occupational and public exposures (effective dose) associated with the 

normal transport of radioactive material have -with few exceptions -been 

consistently in the range of or below of 1 mSv/yr (100 mrem/yr) for transport 

workers and well below of 0,05 mSv/yr (5 mrem/yr) for the general public 

(critical group individuals) for all major transport activities and categories of 

radioactive material. Radiation doses in these dose ranges represent only a small 

 

32 “Normal transport” means transport operations that occur without unusual delay, loss of, or serious damage to a 

radioactive material package, or an accident involving the conveyance. 
32

fraction of the relevant regulatory dose limit for radiation workers and members 

of the public of 20 mSv/yr (2000 mrem/yr) and 1 mSv/yr (100 mrem/yr), 

respectively.33 

In the conclusions to the report the authors state: 

“The comprehensive survey and assessment results confirm that the transportrelated radiation doses, incurred by transport workers and members of the public 

are generally low for all major categories of material and transport activities under 

normal conditions of transport and well below the applicable regulatory dose 

limits (20 mSv/yr for workers and 1 mSv/yr for members of the public).”34

Later they state: 

 “This general observation is according to a European wide assessment study 

performed on behalf of the European Commission broadly consistent with the 

operational experience in other Central European EU Member States.”35

And: 

“The occupational and public radiation exposures data described above are 

believed to reflect well-managed transport and sound management practices and 

may thereby serve as a reasonable basis and guidance material for the 

establishment of an optimised level of radiological protection and safety in 

transport. The radiation exposure data nationally available also indicate that the 

implementation and application of the international transport safety standards, i.e. 

TS-R-1, ensure an adequate level of radiological protection of both workers and 

members of the public for normal conditions of transport and satisfy the radiation 

protection principles of the International Basic Safety Standards (BSS).”36 

As with the NS and French studies cited previously, the results of this study are 

consistent in the findings of low radiation exposures among rail transportation workers. 

 

33 “Assessment of the Radiation Exposures associated with the Transport of Radioactive Material in Germany,” 

G. Schwartz and F. Lange, Gesellschaft fur Anlagen und Realktorsicherheit (GRS) mbH, Schwertnergasse 1, 50667 

Koln, Germany – Proc. IAEA Conference, July 7–11, 2003, Vienna, page 100.

34 Ibid., page 100 

35 Ibid., page 100 

36 Ibid., page 100
33

Conclusions in Response to the Congressional Mandates 

Section 411(a) of the Federal Railroad Safety Improvement Act of 2008 (Pub. L. No. 110-432) 

(RSIA 2008) states: 

“STUDY. The Secretary of Transportation shall, in consultation with the 

Secretary of Energy, the Secretary of Labor, the Administrator of the 

Environmental Protection Agency, and the Chairman of the Nuclear Regulatory 

Commission, as appropriate, conduct a study of the potential hazards to which 

employees of railroad carriers and railroad contractors or subcontractors are 

exposed during the transportation of high-level radioactive waste and spent 

nuclear fuel ... (1) an analysis of the potential application of “as low as reasonably 

achievable” principles for exposure to radiation to such employees ...” 

Response: The data presented in this report include that from the DTS, where the theoretical 

predictions of potential exposure to radiation were based on assumptions of emission levels from 

the packages at the allowed regulatory limit, and real-world exposure assessments in the United 

States and two European countries. Both the DTS theoretical findings and the real-world 

experience indicate that potential and actual exposures are well below the currently established 

permissible levels. All of the current regulatory permissible levels have been established 

recognizing the importance of the “as low as reasonably achievable” (ALARA) principles in 

minimizing exposures. 

Section 411(a)(1) of the RSIA 2008 continues: 

“STUDY... with an emphasis on the need for special protection from radiation 

exposure for such employees during the first trimester of pregnancy ...” 

Response: Aside from the medical and personal privacy issues raised by this question, the realworld exposures found in the three studies cited are well below regulatory limits. According to 

the Center for Disease Control, “Most radiation exposure events will not expose the fetus to 

levels likely to cause health effects. This is true for radiation exposure from most diagnostic 

medical exams as well as from occupational radiation exposures that fall within regulatory 

limits.”37 

Section 411(a)(1) of the RSIA 2008 continues: 

“STUDY ... with an emphasis on the need for special protection from radiation 

exposure for such employees ... or who are undergoing or have recently 

undergone radiation therapy.” 

Response: Without considering the medical privacy issues raised by this question, the realworld exposures found in the three studies cited are very low. However, the medical and health 

 

37 Centers for Disease Control: http://www.bt.cdc.gov/radiation/prenatalphysician.asp (April 2010) 
34

implications of someone undergoing radiation therapy can be extremely varied, and the 

interaction with occupational exposures cannot be presumed given the complexity of the medical 

procedures and types of therapy available in today’s medical practice. 

Section 411(a)(2) of the RSIA 2008 continues: 

“STUDY... the feasibility of requiring real-time dosimetry monitoring for such 

employees.” 

Response: Requiring real-time dosimetry for railroad or contractor employees involved in these 

activities does not appear to be warranted based on the exposures documented to date, nor by the 

very infrequent occurrence of these events. 

Section 411(a)(3) of the RSIA 2008 continues: 

“STUDY... the feasibility of requiring routine radiation exposure monitoring in 

fixed railroad locations, such as yards and repair facilities.” 

Response: The use of dedicated trains minimizes the dwell-time of trains carrying HLRW/SNF 

in fixed locations such as yards and repair facilities. In addition, significant monitoring of the 

packages at the shipping point, required by regulation, along with the known shielding properties 

of the packages, would make monitoring in these locations redundant and would serve no 

practical purpose. 

Section 411(a)(3) of the RSIA 2008 continues: 

“STUDY ... a review of the effectiveness of the Department’s packaging 

requirements for radioactive materials.” 

Response: The DTS was used to establish theoretical levels of exposure to various populations 

potentially exposed to radiation during rail transportation of HLRW/SNF, based on the 

assumption that the shielding afforded by the packages would at least meet the limits established 

in the regulations. The real-world levels measured by NS during actual shipments indicate that 

the packaging far exceeds the minimum requirements, thus providing an extra margin of safety 

for the employees as well as the general public. 

Section 411(b) of the RSIA 2008 states: 

REPORT. Not later than 18 months after the date of enactment of this Act, the Secretary 

of Transportation shall transmit a report on the results of the study required by subsection 

(a) and any recommendations to further protect employees of a railroad carrier or of a 

contractor or subcontractor to a railroad carrier from unsafe exposure to radiation during 

the transportation of high-level radioactive waste and spent nuclear fuel ...” 

Response: Based on the findings of this study, it does not appear that any such 

recommendations are necessary at this time. 
35

Section 411(c) of the RSIA 2008 states: 

“REGULATORY AUTHORITY. The Secretary of Transportation may issue 

regulations that the Secretary determines appropriate, pursuant to the report 

required by subsection (b), to protect railroad employees from unsafe exposure to 

radiation during the transportation of radioactive materials.” 

Response: The Secretary of Transportation does not believe that any regulatory action is 

necessary at this time to further protect railroad employees from unsafe exposure to radiation 

during the transportation of radioactive materials. 
36

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A-1 

Appendix A: 

AAR Circular No. OT-55-J–Recommended Railroad Operating Practices 

for Transportation of Hazardous Materials 
A-2 
A-3 
A-4 
A-5 
A-6 
A-7 
A-8 
A-9 
A-10 
A-11 
A-12 
A-13 
A-14 
A-15 
A-16 
A-17 
A-18 
A-19 
A-20 
A-21 
A-22 
B-1 

Appendix B:

Thermoluminescent Dosimetry 

Radiation absorbed dose is measured using different instruments, one of which is the 

thermoluminescent dosimeter (TLD). This is a simple explanation of how the TLD works: 

When ionizing radiation interacts with any material, some or all of the energy is deposited in that 

material. The energy interacts with the atoms in the material, causing some to lose an electron—

called ionization—and results in the formation of a charged atom, called an ion. 

Thermoluminescence (TL) is the ability of some materials to convert the energy from the 

ionizing radiation absorbed to a radiation of a different wavelength, normally in the visible light 

range through the application of heat to the material. Generally the materials that are used for 

this purpose are crystalline in form. Most crystalline materials contain impurities, thus 

producing irregularities within the crystal structure (lattice). The imperfections in the crystal 

lattice act as sites where free electrons from the ionization process can become trapped, locking 

them in the crystal. The crystalline materials most commonly used in TLDs are made of lithium 

fluoride (LiF) and calcium fluoride (CaF), although some other materials can be used for specific 

applications. 

Heating the crystal causes the crystal lattice to release the trapped electrons, thus releasing the 

captured energy from ionization as light. The intensity of the light released in this way is 

measured using a very sensitive device based on photomultiplier tubes. The number of photons 

is then counted and is proportional to the amount of energy deposited in the crystal. 

Sophisticated TLDs have up to four identical crystals mounted on a card with filters made of 

different materials (for example, plastic, aluminum, copper, etc.), and thicknesses placed in front 

of each of the crystals. The filters help determine the type and energy of the incident radiation 

since different filter material reduces the amount of ionizing radiation getting to the crystals 

differently. The automated reader then heats the four crystals simultaneously and light output 

from each crystal is read out separately. Dose calculating algorithms are applied to the readings 

from the crystals to calculate the radiation dose to the individual wearing the dosimeter. 

For situations where a specific type of radiation is expected or the measurement of one type is 

more “important,” filter materials of different types and thickness can be tailored along with 

calibration procedures using specific sources for the type of radiation of concern to ensure the 

accuracy of those specific measurements. The tailoring of filter materials is also useful for 

estimating exposures to different parts of the body, e.g., skin and shallow tissue, lenses of the 

eyes, and deep body tissue doses. 

The minimum reportable dose is 10 millirem for gamma radiation and x-rays. This is the 

smallest dose that can be measured reliably and accurately. 

Once the TLD is put through the reading process, the crystals are essentially “renewed” since the 

absorbed energy is released by the reading process, and the TLD can be used again.